A known ultrasonic sensor is provided at a flip-up rear hatch (i.e., back door) that opens and closes via a pivot shaft provided at the rear top of a vehicle so that a surface of the back door is included in a detection range of the sensor. The ultrasonic sensor detects an object or an obstacle in the vicinity of the vehicle, i.e., performs an obstacle detection by intermittently transmitting an ultrasonic wave, which hits the obstacle to thereby generate a reflected wave as a reflection of the ultrasonic wave. An example of an opening and closing control system for a vehicle door equipped with such ultrasonic sensor is disclosed in JP2007-138566A (which will be hereinafter referred to as Reference 1). According to the opening and closing control system disclosed in Reference 1, an ultrasonic sensor is provided at a back door for a vehicle to transmit an ultrasonic wave so that a detection range of the sensor extends substantially along a surface of a vehicle body (specifically, the back door). The ultrasonic sensor detects a distance therefrom to an object or an obstacle and a direction where the obstacle is present based on a time interval between the ultrasonic sensor transmitting the ultrasonic wave and receiving a reflected wave from the obstacle. When the distance to the obstacle detected by the ultrasonic sensor is within a predetermined range, the opening and closing control system controls, for example, the back door to stop opening or closing. Because the ultrasonic sensor moves along with the back door when it opens or closes, an edge portion of the back door that is most away from the vehicle body upon opening and closing of the back door is constantly included within the detection range of the ultrasonic sensor. Accordingly, the opening and closing control system successfully performs the obstacle detection when the back door opens or closes, thereby opening or closing an opening portion of the vehicle based on the detection result of the ultrasonic sensor.
JP2007-139570A (which will be hereinafter referred to as Reference 2) discloses a sensor unit used for such opening and closing control system for a vehicle, an obstacle detection system, and the like. The sensor unit disclosed in Reference 2 includes a sensor head arranged next to a license plate lamp of a back door for a vehicle. In addition, JP2007-057527A (which will be hereinafter referred to as Reference 3) discloses an ultrasonic sensor of which sensor head is arranged in the vicinity of a hinge provided at an upper portion of a back door, at a vehicle emblem of the back door in view of design, and the like.
A position where the sensor head is arranged according to Reference 2 is determined, focusing on a routing of wiring, ease of connection with CAN (Controller Area Network), and the like. In addition, a position where the sensor head is arranged according to Reference 3 is determined so that the detection range of the ultrasonic sensor extends along a surface of the back door or determined in view of design. However, none of the above References discloses a method for arranging the sensor head at an appropriate position by excluding a small aboveground structure such as a curbstone and a wheel stopper provided on a road, a parking surface, and the like from the detection range of the ultrasonic sensor while securing the detection range of the ultrasonic sensor. Further, based on the aforementioned standpoint, none of the above References includes considerations or proposals on the arrangement of the sensor head at the appropriate position.
A need thus exists for an ultrasonic sensor which is not susceptible to the drawback mentioned above.